The use of inorganic whiskers and fibers to reinforce glasses, glass-ceramics, sintered ceramics, plastics, and metals has long been practiced. Commonly, the term whiskers has been applied to elongated, single-crystal fibers. In general, whiskers have been described as having a thickness less than about 100 microns with a length-to-diameter ratio of at least 100.
Whiskers have found extensive use as reinforcing agents in various matrices because of their inherent shape, high modulus of elasticity, and high tensile strength. To illustrate, when dispersed in a crystalline matrix, the anisotropic, fiber-like configuration of whiskers strongly inhibits their movement to the grain boundaries of the crystals, thereby significantly improving the creep resistance of the material. Also, the high elastic modulus and tensile strength of many different whiskers enables them to produce composite products demonstrating superior strength-to-weight and stiffness-to-weight properties. For example, whiskers prepared from very stiff, low density covalent compounds such as carbides, nitrides, and oxides can exhibit elastic moduli higher than most metals and are often many times stronger than steel, when considered in proportion to their weight.
In contrast to whiskers, fibers are generally deemed to be multicrystalline or amorphous. Extensive study to understand the basic means underlying the strengthening improvement to composite bodies imparted by the fibers has indicated the mechanism to be that of load transfer by the matrix to the fibers through shear. This load transfer takes place near the ends of the fibers within a distance of a few fiber diameters.
The basic strengthening mechanism is believed to be the same in whisker-containing composites, but the amount of load transferred by the matrix to the whiskers is dependent upon the length and aspect ratio of the whisker. Hence, the whisker may not be loaded to the breaking stress and, consequently, full advantage cannot be taken of its reinforcing capability. In addition to the length and aspect ratio of the whisker, orientation of the whisker with respect to the applied stress and the stress concentrations at the ends of the whisker result in lower strength than would be possible with fibers. Accordingly, whisker reinforced composites will typically manifest lower mechanical properties than unidirectionally-oriented, continuous fiber composites fabricated from like constituents (when properties are measured along the fiber axis). Whisker-containing composites possess an advantage, however, over the continuous fiber-containing composites in that they are macroscopically isotropic.
Glass-ceramics had their genesis in U.S. Pat. No. 2,920,971. As is explained therein, glass-ceramics are prepared through the controlled in situ crystallization of precursor glass bodies. Production of glass-ceramic articles commonly comprises three basic steps: (1) a glass batch of a desired composition is melted; (2) the melt is cooled to a temperature at least within and, normally, below the transformation range of the glass and a glass article of a predetermined configuration shaped therefrom; and (3) the glass article is exposed to a predetermined heat treatment schedule to cause the glass to crystallize in situ. Customarily, temperatures greater than the annealing point of the glass will be employed in the crystallization heat treatment, with temperatures approaching and exceeding the softening point of the glass not being unusual.
SiC fibers and whiskers have been demonstrated as reinforcing agents in numerous metal and non-metal matrices. For example, U.S. Pat. No. 4,324,843 records the formation of SiC fiber reinforced glass-ceramic composite bodies wherein the glass-ceramic matrix is selected from the composition systems of aluminosilicate, lithium aluminosilicate, magnesium aluminosilicate, and combinations thereof. U.S. Pat. No. 4,464,475 discloses the production of SiC fiber reinforced glass-ceramic composite bodies wherein barium osumilite constitutes the predominant crystal phase. U.S. Pat. No. 4,464,192 describes the preparation of SiC whisker reinforced glass and glass-ceramic composite bodies wherein the glass-ceramic matrix is selected from the group of lithium aluminosilicate, magnesium aluminosilicate, aluminosilicate, and combinations thereof.
The above matrices are asserted to be suitable for use temperatures in the range of about 1000.degree.-1300.degree. C. Above that temperature range those compositions are not refractory enough to provide a viscosity sufficiently high to transfer load to reinforcing fibers and whiskers. Consequently, the matrix deforms excessively and the composite suffers loss of load-bearing ability.